This invention relates to thin film resistance thermometer devices (RTD's), and more particularly to thin film RTD chips used to construct the complete resistance thermometer devices where the chips involved are electrically non-conducting support material on which is deposited a thin metallic film.
RTD's are used to measure temperature by relating the device's resistance to the temperature. The resistance increases in an approximately linear manner with slope of .alpha.. Alpha is also referred to as the temperature coefficient of resistance (TCR) of the device, and for platinum RTD's it is common practice to define the TCR as the relative resistance change per degree measured over the interval 0.degree. C. to 100.degree. C. Of great value is the fact that platinum RTD's can be made having the same calibration, that is the same ice point resistance and TCR and can thus be used interchangeably. A process for making a thin film RTD must therefore be able to produce RTD's which follow the same calibration interchangeably.
In the past it has been common practice to attempt to establish the TCR of RTD's of thin film construction in a similar manner as was used to control the TCR of wire wound RTD's, namely by the addition of impurities to the metal being used or by the control of the concentration of defects in the film by heat treating films during and after their processing.
It is well known that the higher the concentration of impurities and the higher the concentration of defects in an elemental metal, the lower the value of TCR. Thus, special alloys of platinum or special defect controlling processes have in the past been used to provide the desired TCR. Usually the temperature coefficient of resistances desired has been the industrial standard, namely 0.00385(.degree.C.).sup.-1, which is slightly below the temperature coefficient of resistance for very thick films or wire of pure, low defect platinum, namely 0.00392(.degree.C.).sup.-1.
In other prior art methods for obtaining a desired TCR, the film has been deposited by means of RF sputtering on a substrate and including oxygen in the sputtering gas, using bias sputtering and annealing at temperatures greater than 800.degree. C. after deposition as set forth in U.S. Pat. No. 4,103,275 issued to Diehl et al on July 25, 1978. In the Diehl patent control of these variables allows a low enough level of the impurity and defect concentration of the platinum film so that a desired value of TCR is obtained.
In addition the Diehl patent describes how a thin film has been deposited on a substrate having a greater thermal coefficient of expansion than the material of the film. In the Diehl patent there is described the method for effectively increasing the TCR of the film in an attempt to bring it up to that of the industrial standard of 0.00385(.degree.C.).sup.-1. It will be noted from the examples given in the Diehl patent that the device did not produce the standard TCR but instead produced a higher value. Thus the Diehl structure does not appear to be capable of obtaining a predetermined TCR with a useful degree of accuracy.
The control of the variables which contribute to lowering the TCR is very difficult and is reflected in the fact that very few techniques are available to deposit metal films with a controlled TCR value. It is the object of this invention to provide a thin film platinum resistance thermometer structure with a predetermined TCR and a method for making such a device.